1. The Field of the Invention
The present invention relates generally to methods and apparatus for measuring levels of chemical compounds located in the anatomy of the eye, and more specifically to methods and apparatus for measuring such compounds in the eye for assessing the risk of suffering diseases of the eye.
2. The Relevant Technology
Vision, or sense of sight, is one's ability to perceive the form, color, size, movement, and distance of objects, by way of complex anatomy generally termed the eye. Vision occurs when light passes through the eye and is absorbed by the sensitive cells of the retina in the back of the eye. Specifically, light enters the cornea of the eye, passes through the pupil, and then through the lens, from which it is projected to the retina. Human vision in particular is sensitive for light in the visible spectrum, which is from approximately 380-720 nanometers in wavelength.
In order to actually see an image, the lens of the eye must bring the image into focus on the retina. Clear vision, or visual acuity, refers to the sharpness of the image and depends in part on the capability of the lens. The portion of the retina in which visual acuity is the greatest is called the fovea centralis. In the fovea centralis, light falls directly on individual photoreceptors, the sensory cells that respond electrically to light, whereas light in other regions of the retina must pass through several layers of nerve cells before reaching the photoreceptors.
Surrounding the fovea centralis is an area called the macula. The macula is less efficiently shielded from incoming light than other retinal tissue because there are fewer cell layers lying between the incoming light and the photoreceptors. The macula is typically yellow in color due to the presence of a high concentration of the carotenoids lutein and zeaxanthin. These carotenoids, which are normal constituents of a healthy diet, have been shown to offer statistically significant protection against age-related macular degeneration, the leading cause of irreversible blindness among the elderly in the United States. Lutein and zeaxanthin are actively concentrated in the macular tissue where it is believed they screen out phototoxic short wavelength visible light and act as free-radical scavenging antioxidants. It is believed that insufficient levels of these carotenoids permit photic and oxidative damage of the macular tissue.
It is possible to detect the levels of the macular carotenoids. One technique measures carotenoid levels in post-mortem eye tissue utilizing conventional biochemical means such as high performance liquid chromatography (HPLC) and visible absorption spectrophotometry. However, this technique clearly suffers from the absence of any value for use in connection with live subjects.
The conventional technique for non-invasive measurement of human macular carotenoids is a "subjective psycho-physical flicker photometric test" which involves color intensity matching of a light beam aimed at the fovea with one aimed at the parafoveal area of the retina. This technique is time-intensive and requires both highly sophisticated optical apparatus and highly skilled technicians. In addition, the patient must be alert, cooperative, and have relatively good visual acuity. Such requirements limit the usefulness of this technique for assessing macular carotenoids in an elderly population at risk for age-related macular degeneration, the very essence of which is a reduction in visual acuity. Elderly patients may face added frustration in an attempt to communicate the necessary information to the technician. Clearly, the conventional technique is not conducive for testing the levels of macular carotenoids in non-communicative patients and animals.